Long term vent structure

ABSTRACT

A vent structure for reliable long term operation at high temperatures is fabricated by annealing a tubing of predetermined length and size, filling the tubing with fine alumina particles, flattening and coiling and pressing an upper portion of the tubing at predetermined pressures, annealing the formed tubing, and installing a particulate filter in the undeformed lower portion of the tubing. For use in venting helium generating reactor control pins located under hot molten sodium, a vent assembly including the vent structure and providing an air lock between the outer molten sodium and the vent is affixed to the upper end of each control pin.

This is a division, of application Ser. No. 296,680 filed Oct. 11, 1972.

BACKGROUND OF THE INVENTION

My present invention pertains generally to venting devices. Moreparticularly, the invention relates to a very effective and practicalvent structure for use in the control pins of a liquid metal fastbreeder reactor (LMFBR), for example, and to a novel method offabricating the vent structure.

As is well known, helium (He) is generated in the LMFBR control pins asthe result of a neutron, alpha (n, α) reaction on boron (B). This heliumis generated in each pin up to the rate of 3 cc/hr or 72 cc/day volumeat standard temperature and pressure (STP). In all of this generated gasis released from the boron carbide (B₆ C) matrix of a control pin, theresulting 26 liters per year poses a serious containment and pressureproblem using closed control pins, especially since sudden onsets ofextreme pressure could develop during the reactor power cycles.Conversely, the use of open control pins allows the boron carbide tocome into direct contact with the high temperature sodium (Na) liquidmetal coolant with the resulting attack thereon and likelihood of boroncontamination of the coolant.

SUMMARY OF THE INVENTION

Briefly, and in general terms, my invention is preferably accomplishedby providing a vent structure, including a ductile and relatively thinwall tubing having an undeformed normally lower portion and a flattenedand folded (doubly coiled and pressed) normally upper portion, in amolten sodium coolant reactor gas generating control pin, for example,to release its generated gas normally at a relatively low flow rate suchthat the control pin (gas container) is maintained in a slightlypressurized state to prevent backflow of molten sodium or vapor andconsequential contamination thereof. The formed (flattened and folded)upper portion of the vent structure preferably includes a thin film orlayer of fine alumina particles between the flattened opposing facesinside the tubing, to prevent the vent from sintering together due tograin growth across the faces occurring at high temperatures and longterm operation. The vent structure preferably further includes aparticulate filter in the undeformed lower portion of the vent tubing.

The method of fabricating the vent structure includes the steps, amongothers, of annealing a metal tubing of predetermined length and size ata predetermined temperature for a predetermined period, uniformlyfilling the tubing with fine alumina particles, flattening an upperportion of the tubing and then coiling and pressing such upper portionat predetermined pressures, and again annealing the tubing at apredetermined temperature for a predetermined period. Finally, a porousmetal particulate filter can be press-fitted into the undeformed lowerportion of the tubing. The resultant vent structure is structurallystable and can operate reliably in a high temperature environment over along term, and gas flow thereof is substantially a linear function ofthe applied gas pressure differential up to the point where the elasticlimit of the metal tubing is reached.

For use in venting helium generating reactor control pins located undera corrosive and reactive liquid environment such as molten sodium, avent assembly including the vent structure and providing an air lockbetween the outer molten sodium and vent structure is attached to eachpin to ensure that liquid sodium does not directly contact the vent. Thevent assembly includes an adapter plug mounting the lower end of thevent structure, cover structure attached to the plug and forming an airlock chamber housing the part of the vent structure above the plug, anda baffle made of porous metal provided in the chamber space between theupper portion of the vent structure and a number of small bleed holes inthe cover structure. The adapter plug is shaped to be suitably attachedto the upper end of a control pin, and the baffle allows free passage ofgas while preventing the passage of particulate matter and impeding thepassage of molten sodium. The possibility of sodium backflow isvirtually eliminated by use of the air lock and the anti-splash porousmetal baffle.

BRIEF DESCRIPTION OF THE DRAWINGS

My invention will be more fully understood, and other advantages andfeatures thereof will become apparent, from the following description ofan exemplary embodiment and method of the invention. The description isto be taken in conjunction with the accompanying drawings, in which:

FIG. 1 is an elevational view, shown in section, of a vent assemblywhich is to be affixed to the normally upper end of a reactor controlpin;

FIG. 2 is a fragmentary elevational view, shown in section and enlarged,of a vent structure constructed according to this invention;

FIG. 3 is a graph showing a plot of helium pressure versus helium flowrate of the vent structure;

FIG. 4 is a graph showing plots of vent temperature versus helium flowrate of two vent structures having nominally different helium releaserates;

FIG. 5 is a graph showing plots of thermal cycles versus helium flowrate at the low and high temperature limits of the thermal cycles;

FIGS. 6A, 6B, 6C, 6D and 6E illustrate certain main steps in a method offabricating the vent structure; and

FIG. 7 is a flow chart illustrating a process of producing the ventassembly.

DESCRIPTION OF THE PRESENT EMBODIMENT AND METHOD

In the following description and accompanying drawings of anillustrative embodiment and method of my invention, some specificdimensions and types of materials are disclosed. It is to be understood,of course, that such dimensions and types of materials are given asexamples only and are not intended to limit the scope of this inventionin any manner.

FIG. 1 is an elevational view, shown in section, of the upper ventassembly 10 of a control pin 12 which is used, for example, in a liquidmetal (sodium) cooled fast breeder nuclear reactor (not shown). Thecontrol pin 12 and its upper vent assembly 10 are normally fullyimmersed deeply in (under approximately 8 feet of) the molten sodiumreactor coolant operating at temperatures of about 900° to 1,100°F. Eachcontrol pin 12 contains a series of boron carbide pellets (not shown)and is a source of generated helium. The vent assembly 10 is, of course,hermetically attached to the upper end of the control pin 12 andgenerally includes a vent adapter plug 14, baffle 16, air lock tube 18,air lock cap 20, and vent structure 22.

The vent plug 14, air lock tube 18, and air lock cap 20 are preferablymade of a material similar to that of the control pin 12 structuralmaterial such as Type 316 stainless steel, which is compatible with ahot sodium (liquid and vapor) environment. Baffle 16 is made of a porousmetal which is resistant to attack by hot sodium. A felt or foam metalcan be used and, in the exemplary vent assembly 10, a Type F-315 nickelfelt metal produced by Huyck Metals Corporation was used. This materialis about 20 percent dense and allows free passage of gas whilepreventing the passage of particulate matter and impeding any passage ofmolten sodium.

The air lock tube 18 has, for example, four vent or bleed holes 24 whichcan be 0.0135 inch in diameter equiangularly spaced 90 degrees about theair lock tube at a predetermined distance above the lower end thereof.The vent plug 14 has an axially drilled central hole 26 and the baffle16 also has an axially punched central hole 28. The punched baffle 16 ispacked on the vent structure 22 against the upper surface 30 of ventplug 14 which is joined at its lower end 32 to the lower end of the ventstructure by a standing lip electron-beam weld for maximum cleanlinessand minimal disturbance of the surrounding metal.

When the lower end of the air lock tube 18 is welded to flange 34 of thevent plug 14, the vent holes 24 are located slightly above the uppersurface 30 of the vent plug and directly adjacent to the lower sidesurface of the baffle 16. Welding of the air lock cap 20 to the upperend of the air lock tube 18 forms an air lock chamber 36 containingcover gas which provides an "air lock" effect over the vent structure 22such that liquid sodium does not directly or normally contact the uppervent end 38. The possibility of sodium backflow is virtually eliminatedby use of the air lock chamber 36 and the anti-splash porous metalbaffle 16 in the vent assembly 10 of control pin 12.

The air lock chamber 36 is a plenum chamber which is made large enoughto provide a sufficient reservoir of gas that prevents liquid sodiumwhich might enter the lower bleed holes 24 and into the baffle 16, as inthe event of any sudden fluctuation (loss) in gas pressure due to atemporary reactor temperature change (drop), from ever reaching the ventopening in the upper vent end 38; and of the vent structure 22. Ofcourse, the sodium is subsequently forced out of the chamber 36following the temperature change as the gas pressure therein builds upto equalize with the environmental (8 feet of liquid sodium) pressure.

The bleed holes 24 are made adequately small so that they will not admitthe surge of liquid sodium into the air lock chamber 36, which surge canoccur when the control pin 12 is first immersed in the sodium. On theother hand, the bleed holes 24 are made adequately large mainly forconvenience of drilling very small holes in stainless steel with thepresently available drills and methods. In any event, the combined sizeof the bleed holes 24 must be about equal or (and are vastly) largerthan the effective size of the vent discharge opening in the upper ventend 38 of the vent structure 22, and allows gas to escape from thechamber 36 at about the rate that it is being released from the ventstructure.

The vent assembly 10 has general overall dimensions of length A,diameter B, and an approximate gas space length C (connected throughvent holes 24 to the exterior). Illustrative values for these dimensionsare A of 1.80 inches, B of 0.435 inch maximum), and C of 0.95 inch, forexample. This vent assembly 10 is, of course, to be welded to the upperend of the stainless steel tube (control pin 12) having a 0.395 inchinside diameter which accommodates the lower portion of vent plug 14.Other dimensions of the vent assembly 10 can be proportionatelyestimated adequately from the elevational view of FIG. 1 and, whileapproximate, will suffice for most purposes. The vent has a gas flowrate sufficient to maintain pressure in the air lock chamber 36 and incontrol pin 12 to prevent backflow of sodium and any possiblycontaminating outside cover gas, respectively.

FIG. 2 is a fragmentary elevational view, shown somewhat enlarged insection, of the vent structure 22. The vent structure 22 includes a thinwall tubing 40 having an undeformed lower portion 42, and a flattenedand folded (coiled or rolled and pressed) upper portion 44. Theflattened upper portion 44 is preferably folded at least twice, asshown. A particulate filter 46 is provided in the lower portion 42 ofthe tubing 40, and a film or layer 48 of fine particles or powder isprovided between the pressed opposing surfaces inside the upper portion44. The tubing 40 can be made of any material sufficiently ductile tosurvive the flattening, coiling and pressing operations withoutcracking, and there are appropriate materials readily available foroperation or use under a very wide range of environmental andtemperature conditions.

In the interests of compatibility and uniformity, the tubing 40 can bemade of the same material as the control pin 12 structural material.Thus, the tubing 40 can be made of Type 316 stainless steel having aninside diameter D and an outside diameter E as indicated in FIG. 2.Likewise, the particulate filter 46 can be made of Type F-315 nickelfelt metal similar to that of baffle 16 (FIG. 1) and having a length F.The cylindrically shaped filter 46 can be installed in the lower portion42 by press-fitting it into the tubing 40 to a position approximately asillustrated. Exemplary dimensions for the inside diameter D, outsidediameter E, and length F are respectively 0.093, 0.125, and 0.250 inch,for example.

The formed upper portion 44 of the exemplary vent structure 22 has alength G and a thickness H as indicated in FIG. 2. The film or layer 48can consist essentially of, for example, compacted 0.3 micron diameteraluminum oxide (Al₂ O₃) particles. This particle size could be increasedto as large as 1 micron or reduced to as small as 0.05 micron forsatisfactory operation. A particle size larger than 1 micron wouldprovide individual support points during forming (flattening andpressing) of the upper portion 44 and which may prevent the attainmentof low gas flow rates. Of course, a particle size smaller than 0.05micron may not prevent eventual grain growth across the proximate facesof a flattened tube at high temperatures. Thus, in regular long term andhigh temperature operation, it is possible for the formed upper portion44 of the vent structure to sinter together. Note that the formed ventstructure 22, with or without the layer 48 of alumina particles, can beused independently as a gas flow metering device in various applicationsfollowing suitable flow calibration thereof.

Illustrative dimensions for the length G and thickness H arerespectively about 0.350 and 0.092 inch, for example. The thickness ofthe film or layer 48 can be approximately 0.005 inch, when pressed,although the layer thickness in the exemplary vent structure 22 can begenerally between 0.003 and 0.007 inch for satisfactory operation. Exactpressing parameters and anneal conditions are functions of theparticular size and wall thickness of the tubing 40 used and the desiredgas flow rate. The method of fabricating the vent structure 22 will bedescribed later, following a discussion of certain test results thereof.

FIGS. 3, 4 and 5 are graphs showing respective plots of helium pressureversus helium flow rate of the vent structure 22, vent temperatureversus helium flow rate at one atmosphere pressure differential, andthermal cycles versus helium flow rate wherein the cycle period was twominutes from high temperature to low temperature. The curve 50 of FIG. 3illustrates that a substantially linear relationship exists in theresponse in helium flow rate of the vent structure 22 to change inhelium pressure. As indicated in FIG. 3, helium flow rate drops linearlyas the driving pressure decreases.

In testing the vent structure 22, it was thermally cycled 60 timesbetween 375° and 1130°F wherein the cycle period was two minutes fromhigh temperature to low temperature and 2 minutes back to hightemperature. Curve 52 of FIG. 4 illustrates helium release rates of thevent structure 22 as a function of its temperature while maintaining apressure differential of one atmosphere for the vent structure. Flowrate is, of course, lower at the higher temperatures since there areless gas molecules in a fixed volume at such temperatures with themaintained pressure differential. Curve 54 of FIG. 4 illustrates heliumrelease rates of another vent structure as a function of its temperatureat a pressure differential of one atmosphere, the vent structure beingsimilar to vent structure 22 but formed to provide a nominally higherflow rate.

The effect of thermal cycling between 375° and 1130°F of the ventstructure 22 is illustrated by the curves 56 and 58 of FIG. 5. Curve 56depicts the helium flow rate at the low temperature of 375°F and curve58 depicts such flow rate at the high temperature of 1130°F. After thevent structure 22 has been thermally cycled 60 times between thesetemperatures, it can be seen that there are slight but not functionallysignificant changes in performance thereof.

With the helium generation rate per control pin 12 (FIG. 1) atapproximately 3 cc/hr, the vent structure 22 provides an average heliumdiffusion rate of about 0.61 cc/hr at one atmosphere pressuredifferential after 60 thermal cycles (FIG. 5). This allows a heliumpressure of approximately 4 atmospheres inside the control pin 12 toprovide a two-fold advantage. First, pressurized helium has a higherthermal conductivity to dissinate radiation heating in the control pinand, second, the pressure minimizes any possibility of back diffusion ofeither sodium vapor or cover gas into the control pin. The positivehelium pressure maintained in the control pin 12 by the vent structure22 can drop by a factor of about 2 without compromising the sodium sealprovided.

FIGS. 6A through 6E illustrate certain main steps in the method offabricating the vent structure 22. In FIG. 6A, following ultrasonicinspection of a Type 316 stainless steel tubing 40 of 0.125 O.D. and0.016 inch wall, the tubing is cut to a predetermined length and vacuumannealed 15 minutes at 1900°F, for example. Austenitic stainless steelis virtually unaffected by pure hot sodium at temperatures below 1000°F.In the region between 1,000° and 1500°F, however, there is evidence ofsome measurable attack, particularly at the grain boundaries. Based onthis constraint, the vent tubing 40 wall thickness was selected to be0.016 inch to provide a safety factor of at least 3 over the deepestsodium penetration observed at 1500°F. The annealed tubing 40 is thenuniformly filled loosely with alumina particles of a generallypredetermined size as, for example, 0.3 micron diameter. Tamping isunnecessary and it is not critical that the tubing 40 be very uniformlyfilled. A light shaking of the tubing 40 during filling ordinarilysuffices. In fact, one way to fill the tubing 40 is simply to push itinto a jar of alumina.

A predetermined length of the filled tubing 40 is then flattened in, forexample, a Carver hydraulic press at a predetermined flatteningpressure. The flattened tubing 40 is illustrated in FIG. 6B. Theflattened portion of tubing 40 is next formed by rolling it into atwo-bend spiral as shown in FIG. 6C, and a final pressing operation isperformed thereon at a predetermined pressing pressure. This producesthe configuration illustrated in FIGS. 6D and 6E and which creates aflat tortuous path for the escaping gas. The compacted aluminathroughout the vent wall interface eliminates the operational sinteringtendency due to grain growth across the faces of the flattened tubing 40at high temperatures. The formed tubing 40 is then preferably vacuumannealed 15 minutes at 1900°F again. A particulate filter 46 is finallypress-fitted into the formed tubing 40 as indicated in FIG. 6E tocomplete the vent structure 22. The formed tubing 40 is preferablyannealed particularly where the vent structure 22 is used in hightemperature (600°C or 1112°F) operation because flow rate doubles in thevent structure going from a stressed to annealed condition and operatingat the high temperature of 600°C, the vent will self-anneal over aperiod of 3 weeks.

The alumina or aluminum oxide powder is compatible with any vent tubingmaterial and is uniquely suited for operation in the presence of hotsodium vapor. It is an extremely stable oxide and is the only commonlyprocessed oxide resistant to attack by hot sodium and sodium vapor. Asmentioned previously, however, exact pressing parameters (and annealconditions) are a function of the particular size and wall thickness ofthe vent tubing used and the desired gas flow rate. In the flatteningand pressing operations required on the particular vent tubing 40 of0.125 inch O.D. and 0.016 inch wall, air flow rate at 15 psidifferential pressure is illustratively varied according to flatteningand pressing pressures as indicated below.

    ______________________________________                                        Flattening Pres-                                                                           Pressing Pres-                                                                              Air Flow Rate                                      sure (psi)   sure (psi)    (at 15 psi ΔP)                               ______________________________________                                        2000         2000          6       cc/hr                                      2500         2500          3       cc/hr                                      4000         4000          1       cc/hr                                      5000         5000          0.5     cc/hr                                      ______________________________________                                    

It is noted that gas flow rate through the vent structure 22 is alsodependent upon the nature of the gas. As vents are made for lower andlower flow rates, this difference becomes more significant as indicatedbelow.

    ______________________________________                                        Air Flow Rate    Helium Flow Rate                                             (cc/hr)          (cc/hr)                                                      ______________________________________                                        6                9                                                            3                6                                                            1                2.5                                                          0.5              1.5                                                          0.1              0.4                                                          ______________________________________                                    

By changes in pressing pressures and techniques, the vent structure 22can be fabricated for any required flow down to, for example, 0.0036cc/hr helium at one atmosphere pressure differential. Also, flow throughthe vent structure 22 can increase greatly in case of a sudden increasein helium pressure within the control pin 12. Thus, the vent structure22 effectively acts to relieve pressure transients in the control pin 12to prevent any possible ruptures thereof and then returns by naturalspringback to normal operation if the elastic limit of the formed tubing40 has not been exceeded. The illustrative vent structure 22 has beentested and found good to over 3500 psi, for example. While the doublyfolded, formed tubing 40 does not actually unroll in order to relievehigh increases in pressure, it tends to do so. This was substantiated intesting a vent structure 22 with increasing pressure until it burst at avery high pressure, when the formed tubing 40 did unroll to some extent.

FIG. 7 is a flow chart illustrating a process of producing the ventassembly 10 (FIG. 1). In the process, the Type 316 stainless steeltubing 40 is provided in a step 60, for example. The tubing 40 ischecked for proper dimensional tolerances and ultrasonically inspectedin step 62, so that it can be certified free from flaws. The tubing 40is cut to length and ultrasonically cleaned in Freon, for example, instep 64. The cut tubing 40 is next vacuum annealed 15 minutes at 1900°Fin step 66. The annealed tubing 40 is formed in step 68 according to themethod shown and described above with respect to FIGS. 6A through 6E.The formed tubing 40 is preferably vacuum annealed 15 minutes at 1900°Fas indicated in step 70 and calibrated for flow in the next step 72.Each formed vent is tested and individually characterized for heliumflow in standard cc per hour by measuring actual pressure rise in avacuum system of calibrated volume with helium on the inlet sideprovided at 760 torr. These tests also serve as the flow acceptance testfor each vent.

Because of the production of lithium as a result of the (neutron, alpha)reaction on boron carbide and the possibility of boron carbidedisintegration under irradiation, the unpressed section (lower portion42 in FIG. 2) of tubing 40 can include the particulate filter 46 whichallows free passage of gas. To produce the particulate filter 46, nickelfelt metal can be used as indicated in step 74. Of course, a foamedinstead of felt metal can be used and the metal can be stainless steelinstead of nickel, for example, since such materials are resistant toattack by hot sodium and allow free passage of gas. Following thematerial inspection step 76, a filter disc or discs of appropriate sizecan be punched out from the nickel felt metal in step 78 and the discfilter inspected in step 80.

Baffle 16 can be fabricated from stainless steel foam metal as indicatedin step 82. This baffle material can, of course, be nickel instead ofstainless steel and felt instead of foam metal. indeed, it is usuallyconvenient and preferable that the baffle 16 and filter 46 be made ofthe same identical material. Following material inspection step 84, thefoam metal is punched or otherwise shaped to size and configuration instep 86. The shaped baffle 16 is inspected in step 88. In similarmanner, the vent adapter plug 14, air lock tube 18 and air lock cap 20can be shaped from a Type 316 stainless steel rod provided in step 90and inspected in step 92. The plug 14, tube 18 and cap 20 can bemachined to size and shape in step 94 from the stainless steel rod, andthe finished products inspected in step 96. It is, of course, apparentthat the formed tubing 40, filter 46, baffle 16, adapter plug 14, airlock tube 18 and air lock cap 20 can be fabricated in any electedsequence or all concurrently.

The particulate filter 46 is packed into the formed tubing 40 tocomplete vent structure 22 and the baffle 16 is packed onto the formedtubing in step 98. The adapter plug 14 is installed on the ventstructure 22 against the lower surface of the baffle 16 and is joined atits lower end 32 to the lower end of the vent structure by a standinglip electron-beam weld in step 100. A helium leak check is made of theelectron-beam weld in step 102, and the air lock cap 20 istungsten-inert-gas (TIG) or electron-beam welded to the air lock tube 18with its vent holes 24 (FIG. 1) located at the opposite end away fromthe cap in step 104. The air lock tube 18 is then TIG welded to theadapter plug 14 in step 106 to complete vent assembly 10, and acceptancetests are performed thereon in step 108. Completed assemblies can bepackaged in plastic bags and sealed for delivery.

While an exemplary embodiment and method of this invention have beendescribed above and shown in the accompanying drawings, it is to beunderstood that such embodiment and method are merely illustrative of,and not restrictive on, the broad invention and that I do not desire tobe limited in my invention to the specific constructions or arrangementsor steps shown and described, for various obvious modifications mayoccur to persons having ordinary skill in the art.

I claim:
 1. A vent structure comprising:a tubing of predetermined lengthand size, said tubing including an undeformed normally lower portion anda formed normally upper portion, and said upper portion being flattenedand folded against itself; and means provided in said upper tubingportion for separating its opposing proximate faces to prevent sinteringand similar closures thereof whereby proper venting of fluid flow can bemaintained through said vent structure to produce a reliable, long termoperation, vent structure.
 2. The invention as defined in claim 1wherein said upper tubing portion is folded at least twice in agenerally spiral configuration.
 3. The invention as defined in claim 1wherein said means provided in said upper tubing portion includes afluid-transmissive layer of material for separating said opposingproximate faces to prevent sintering and similar closures thereof.
 4. Avent structure comprising:a tubing of predetermined length and size,said tubing including an undeformed normally lower portion and a formednormally upper portion, and said upper tubing portion being flattenedand folded against itself; and a relatively thin layer of small sizedparticles in said upper tubing portion separating its opposing proximatefaces and preventing them from sintering together.
 5. The invention asdefined in claim 4 wherein said upper tubing portion is folded at leasttwice in a generally spiral configuration.